I have cold air coming in through my air return. Coming in around 50 degrees. It was measured by an infrared camera. I have had an HVAC guy come out and said the duct was sealed properly with metal foil tape, it is insulated, yet still have this tremendous cold air coming in that goes down the stairs and into other bedrooms. It really makes things cold around the house. An instant temp change you feel as you walk down the hall towards the draft.

I know my house is under negative pressure, but drawing in that kind of air. How in the world is the air getting in the duct? We do not even turn on the heat that the air return is attached to. I opened a window near the air return and did not seem to make a difference. I was trying to stabilize the pressure in the home.

I can't say for sure as I do not know what your setup is but, I do know that some forced air systems have a fresh air make up duct that comes directly from the outside and ties into the return air duct. This may be where your getting your cold air from.